This invention relates to anaerobic adhesive compositions, the reaction products of which demonstrate enhanced toughness and resistance to crack propagation. More particularly, the reaction products of the present invention exhibit increased toughness after exposure to thermal shock, while retaining their room temperature performance properties.
Numerous applications employing anaerobic compositions require certain toughness properties to be fully effective. For example, in certain applications where high compressive loads are placed on the cured anaerobic composition such as in load-bearing joint applications, it is useful for the compositions to retain their structural integrity and performance characteristics to properly serve their role. Toughness is often measured by physical testing, such as impact tests, compression tests and resistance to crack propagation tests, among others. Other indications, however, exist which are known to be indicative of toughness. For example, the presence of discreet domains or phase separation of the cured components is considered an indication of enhanced toughness, when one of the phases is known to be more xe2x80x9crubberyxe2x80x9d and less brittle than another phase. Achieving an anaerobic composition which exhibits toughness using either of these toughness indicators involves the balancing of numerous factors and is difficult to achieve. Various attempts at providing anaerobic compositions having enhanced toughness properties have not provided entirely satisfactory results.
One attempt to achieve a toughened anaerobic composition is disclosed in U.S. Pat. No. 4,018,851 to Baccei. This patent provides polymerizable compositions based on prepolymers which are derived from the reaction between vinyl grafted poly(alkylene)ether polyols and organic polyisocyanates. These prepolymers are also (meth)acrylate terminated. The prepolymers are cured by free radical generating initiators, such as peroxy compounds or ultraviolet sensitive compounds. This patent focused on compositions which produced reaction products having a combination of relatively rigid and flexible alternating segments to obtain greater impact resistance. This technology was expanded further in U.S. Pat. No. 4,308,526, also to Baccei. In the ""526 patent, hydroxyalkyl (meth)acrylate or aminoalkyl (meth)acrylate were reacted with the reaction products disclosed in the aforementioned ""851 patent to achieve greater gap filling capability while retaining impact strength.
U.S. Pat. No. 4,295,909, also to Baccei, disclosed anaerobic compositions which contain urethane-acrylate-capped prepolymers based on polybutadiene polyols or polyamines. The polybutadienes introduce a flexibility into the cured product which results in improved thermal, impact and cure-through gap properties. The polybutadiene polyols and polyamine reactants used to form the cured product comprise about 5-150 butadiene units and have at least about 70% of the butadiene units in the 1,4-configuration. These reactants are reacted with either a molar excess of a reaction product of a polyisocyanate with a polyol or just with a polyisocyanate. The product of this reaction is then capped with an hydroxyalkyl (meth)acrylate. U.S. Pat. No. 3,425,988 to Gorman discloses sealant compositions which are the reaction product of an organic polyisocyanate and a monomethacrylate-terminated ester.
None of the aforementioned compositions completely satisfy the need for toughened anaerobic compositions, particularly in applications where high compression loads and thermal shock is prevalent. For example, in certain applications where the composition forms a retaining function, such as the retention of bearings in helicopter rods, a composition which can tolerate high compressive loads for the cycle life of the part, while resisting crack propagation due to thermal shock is greatly needed. None of the aforementioned patents have disclosed compositions which adequately serve these and other needs.